The present invention relates to a rake receiver which rakes reception results of multipaths, and a mobile unit and a base station for a portable telephone system which use such a rake receiver, and more particularly to a reduction of a circuit scale.
In recent years, a portable telephone of the CDMA system in which different PN (pseudo-noise) codes are allocated to users is rapidly developed. Recently, a system in which a PNW code consisting of a combination of short and long codes is used as the PN code has been studied. A short code is defined for each cell. A spread code is generated by multiplying the short code with a long code which is allocated to each user. When a short code has four chips, the chip number of a PNW code is four times that of a PN code constituting a long code.
In the case where such a code is used, even when any short code is selected in a base station, the communication channel of a cell never coincides with that of another cell.
In the CDMA system, multipaths can be separated from each other in processes of spread and despread. The reception quality can be enhanced by raking reception results of the separated paths. When a PNW code is used, chips are spread in a higher degree, and hence the performance of the rake reception is further improved.
A CDMA portable telephone which performs a rake reception has a plurality of finger circuits which perform demodulation. The finger circuits demodulate received data of different paths, and the demodulated data are raked.
In the CDMA system, furthermore, a pilot symbol is periodically inserted into a transmitted frame. On the reception side, the pilot symbol is detected so as to acquire synchronization, and the acquired synchronization is tracked.
As shown in FIG. 5, a finger circuit of a conventional CDMA portable telephone comprises: a decimating unit 40 which decimates received signals RxI and RxQ in accordance with the acquired synchronization; a despreading unit 42 which despreads OnTimeI and OnTimeQ that are decimated in the decimating unit 40, by using PNW to demodulate the received data; a data path demodulating unit 45 which receives demodulated data DataOnTimeI and DataOnTimeQ that are obtained as a result of the despread, and which decodes the data; a despreading unit 43 which despreads OnTimeI and OnTimeQ by using a long code PN and demodulates pilot data; a pilot carrier tracking unit 46 which receives pilot data PilotOnTimeI and PilotOnTimeQ that are obtained as a result of the despread, and which performs synchronous tracking; a decimating unit 41 which decimates the received signals RxI and RxQ at various timings in order to perform synchronous acquisition; a despreading unit 44 which despreads ELI and ELQ that are decimated in the decimating unit 41, by using a long code PN; and a time tracking unit 47 which acquires synchronization by using PilotELI and PilotELQ that are obtained as a result of the despread.
In the finger circuit, as shown in FIG. 10, the decimating unit 41 decimates the received signals RxI and RxQ after the timing of ELCLK, and outputs ELI and ELQ, and the despreading unit 44 despreads the PN codes of PnI and PnQ into the ELI and ELQ.
In the time tracking unit 47, when a pilot data cannot be detected from the outputs PilotELI and PilotELQ of the despreading unit 44, ELCLK is shifted by one clock. When ELCLK is sequentially shifted by one clock in this way, it is possible to acquire a synchronized clock which enables a pilot data to be detected.
When synchronous acquisition is attained, the clock is input as OnTimeCLK into the decimating unit 40, and the decimating unit 40 decimates the received signals RxI and RxQ from OnTimeCLK to output OnTimeI and OnTimeQ.
The despreading unit 42 despreads OnTimeI and OnTimeQ by using PNW codes of PnWI and PnWQ, and demodulates the received data DATaOnTimeI and DataOnTimeQ. The demodulated data are supplied to the data path demodulating unit 45.
The despreading unit 43 despreads OnTimeI and OnTimeQ with the PN codes of PnI and PnQ and demodulates pilot data PilotOnTimeI and PilotOnTimeQ. The demodulated data are supplied to the pilot carrier tracking unit 46. The pilot carrier tracking unit 46 performs synchronous tracking while adjusting OnTimeCLK supplied to the decimating unit 40, so that the pilot data can be always detected from the outputs PilotOnTimeI and PilotOnTimeQ of the despreading unit 43.
In order to enhance the performance of rake reception, it is required to increase the number of finger circuits and that of paths which are to be subjected to demodulation in the finger circuits. Each finger circuit employed in a rake receiver of the prior art has despreading units which are respectively used for a data path demodulating unit, a pilot carrier tracking unit, and a time tracking unit. As the number of finger circuits is larger, therefore, the number of despreading units is increased, thereby producing a problem in that the circuit scale of the whole of a rake receiver is extremely enlarged.